A galvanometric optical scanner is a limited rotation motor to which a position detector is mounted. Normally the scanner carries a mirror that is used to deflect a beam of light, typically a laser beam. It is common to assemble two such units in series, to deflect light in two orthogonal directions and so address an entire surface. Such a paired unit is commonly referred to as a "two axis scanner" or a "two axis scan head".
Single axis scanners as well as two axis scan heads are used in hundreds of applications, in such categories as laser micro machining, visual communication, artificial extension of vision, material processing, medical procedures, and entertainment.
Frequently, laser scanning is used for extremely precise functions. Examples are satellite to satellite communication, motion picture creation from computer data, silicon microchip repair, laser engraving, and laser guidance for aircraft and missiles. In these and other applications, pointing or tracking precision and stability are critical.
Modern optical scanners are expected to have sub-microradian resolution and stability. The desired stability is frequently equal to the resolution of the instrument, e.g. 1 microradian, over a range of environmental conditions. The critical element that determines these properties is a position sensor, also known as a position transducer, mounted on the shaft of the limited rotation motor. Capacitive transducers are commonly used for such applications and numerous designs have been described, see Miller U.S. Pat. No. 3,517,282 and U.S. Pat. No. 3,577,072, and Foldvari et al, "Capacitive Transducers", Instruments and Controls Systems, November, 1964. A modern position transducer is an analog device with low inertia, often less than 1 g.cm.sup.2, that can resolve better than 1 microradian with a range over 1 radian. These capacitive transducers are derived from the common variable capacitor in which substrates and movable elements may be made of extremely stable and rigid isotropic materials such as PCB or ceramic material, which are comparatively low in cost. Capacitive angular position transducers are preferred for scanning applications because they have low inertia, very high resolution and very small time constant, as well as being comparatively inexpensive to build.
Despite these features, a major shortcoming is lack of stability. The signal, gain and null pointing of capacitive transducers all drift as a function of temperature, humidity or simply over time. The associated decoding electronics are also analog elements and must use extremely stable components. As the associated electronics employ a few dozen critical elements, the stability of each element needs to be an order of magnitude greater than that of the system. This is rarely achieved and never achieved at low cost of construction and verification.
A high quality optical scanner available from Cambridge Technology Inc., Cambridge, Mass., for instance, has resolution under one microradian but, according to Fadvertisement, has null drift of 10 microradians/deg. C, gain drift of 50 PPM/deg. C and short term uncorrected drift of 8 microradians for units equipped with Automatic Gain Control (AGC).
When good stability is imperative, with stability of the same value as the 1 microradian resolution of the system, construction and verification of the units is very demanding. A number of techniques have been proposed in efforts to provide compensation to satisfy the need for high stability.
One approach is exemplified by Rohr U.S. Pat. No. 4,864,295 in which, on the mechanical structure of the position sensing transducer, a secondary capacitor is employed for the sole purpose to measure thermal drift and to use the drift value to correct the error in position signal. Commercial transducers have rarely been built this way. Other designs add a number of elements and plates such as guards to obtain similar effects. Stokes et al, U.S. Pat. No. 5,099,386 is an example.
Another corrective method consists of installing in the field of view, at added expense and inconvenience, a number of optical fiducial elements as reference points, which are detected optically and used to recalibrate the capacitive position sensor. The challenge is to detect beam position with the same resolution that is expected from the scanning system. In pointing applications, such fiducial elements in the field of view are capable of identifying the beam position with suitable resolution, the system typically employing split cell optical sensors and retro-reflectors. Another method is to add vision metrology through the same optical path, employing identifiable features of the object being inspected. In vision metrology applications, the features identified in the field of view are used to recalibrate the analog angular position sensors. Trepanier U.S. Pat. No. 5,400,132 exemplifies the use of retro-reflectors in the field of view. Weisz, U.S. Pat. No. 4,918,284 uses features on the object being viewed, which are inspected for calibration or alignment purposes.
In periodic systems such as those found in most polygon laser printers, the printing plane has installed start-of-scan and end-of-scan split cell optical sensors for pointing and range definition.
An alternative corrective method has been to create an additional optical path, the only purpose of which is for calibration or position definition. Typical of these is the Pell box.TM. of ECRM and the microscopy scanning system of Tsien U.S. Pat. No. 5,296,703.
It is frequently impractical or impossible to install fiducials in the field of view, for instance in laser guiding systems for rocket firing or satellite communication, and in any event these and other proposals add to cost or inconvenience, and, overall, have not been as effective as is desired.
The subject of this invention is a simple and economical design for a stable capacitive position transducer. Without compromising resolution or size or any other feature, this invention offers sensitivity and null stability over a wide range of conditions, equivalent to the sensor's resolution. An additional feature of the concept is the cost benefits derived from the ability to use ordinary electronic elements in the associated circuitry and to eliminate the need for fiducials in the field of view.
While the invention addresses a particular need related to galvanometric systems and limited rotation motors, the invention can have applicability generally to oscillating systems requiring precise positioning.